'94 5.7 Strange EGR issue

[gregs78cam]

OK fellow gearheads, I have a coworker a '94 5.7L and he is having strange EGR symptoms. At very low loads/thottle position, even in P/N, the PCM is showing 100% desired EGR, but 0% actual EGR. I was under the impression that the EGR was a proportional flow not just ON/OFF. Most, if not all top end sensors have been replaced, i.e. TPS, MAP, IAC, EGR solenoid, EGR valve. When I hooked up to it it showed 3 codes, High TPS, EGR, and Knock circuit, some or all may have been from who know when. I cleared all of them to see which ones are actually set when the rough running condition happens, but nothing showed up when it ocurred. I will post back shortly with the 'log. He got it to stay in the rough running condition for an extended time at around 6 minutes.

[EagleMark]

What was the stock bin or at least mask ID?

[gregs78cam]

Stock .bin posted, Mask ID $0D.

It turns out, he may have installed the wrong EGR valve, but the original still tests ok, and ran the same as the replacement.

[EagleMark]

I looked through that entire log waiting for idle but it looks like you need to give it gas at idle to keep it running... your only getting 13.5 volts when driving, look what happens to batterey voltage when your driving and come to 0 MPH and idle at 7:24 in log, you can use the monitor to find exact time... 11.4 volts will not run EFI properly. Load test the battrey, it's bad!

Also ALDL Transmission Diagnostics says enabled?

I didn't know BCC to do exact compare but looks like all EGR settings are stock on bin I picked BHMX 5.7L 4L60E

Watch voltage at idle, if OK turn on lights and? Something is wrong with battrey and or charging system... you can unplug vacuum to EGR valve if it is seated and plug line. If it's seated it will idle and run and drive until it does Code 32 EGR test... then set code...

[gregs78cam]

Actually it idles fine. In all other respects it runs excellent for 223K miles. Problem is light throttle, like rolling through a parking lot, or even at 30+ mph when feathering the pedal it is not nearly as bad but it is there.

[1leg]

This sounds just like the problem i'm having. I switched bins and started leaning out the idle area VE cells and it has improved alot but its still there. I was thinking EGR too. I was planning to plug the egr to test it but forgot to buy vacuum port plug when i was at the store today.

[EagleMark]

That's what all that slow driving was... idle BLM are close, but 50KPA up where your getting most counts are 106.

[gregs78cam]

Well as a test, I burned him a chip with just the following changes in constants:
MIN TPS% for EGR 3.1 --> 10
MIN MPH for EGR 0 -->10
MIN RPM for EGR 800 -->1200

He reported back that it made a HUGE difference, and I my guess is that now when EGR is commanded, the engine is beginning to make a little power and is not affected like it was before. He is going to drive it for a week and let me know.

[EagleMark]

cool. I wonder why though? Could be the motors milage? Or could be the EGR valve issue differant valves one was negitive pressure and one was positive pressure we had a long talk about here?

[gregs78cam]

cool. I wonder why though? Could be the motors milage?

That was one thought that went through my head.


Or could be the EGR valve issue differant valves one was negitive pressure and one was positive pressure we had a long talk about here?

Yea....the one that he took off was a negative pressure, and the one that went on we think is a port EGR(has no letter after numbers) so we are thinking that a bad neg. EGR will operate just like a properly working PORT EGR (will not modulate itself). In this case neither is right for this engine.

As things are now, he can get by until he can get the proper valve, and we will go from there. I will keep this thread updated.


Ported EGR valves (1973 to 1980s). The typical ported vacuum EGR valve consists of a vacuum diaphragm connected to a poppet or tapered stem flow . The EGR valve itself is usually mounted either on a spacer under the carburetor (http://www.aa1car.com/library/carburetor.htm) or on the intake manifold. A small pipe from the exhaust manifold or an internal crossover passage in the and intake manifold routes exhaust to the valve. When vacuum is applied to the EGR valve, it opens. This allows intake vacuum to suck exhaust into the engine. To prevent the EGR valve from opening when the engine is cold, the vacuum line to the EGR valve may be connected to a parted vacuum switch or a computer-controlled solenoid. Vacuum is not allowed to pass to the valve until the engine is warm. EGR isn't needed when the engine is cold, only when it is warm and under load.

Positive backpressure EGR valves (1973 & up). Backpressure EGR valves use exhaust backpressure to vary the point at which they open and their flow rates. On GM cars, they are identified by the last letter on the on top of the valve. A letter "P" indicates a positive backpressure valve, and a letter "N" indicates a negative backpressure valve. Inside a backpressure EGR valve is a second diaphragm that reacts to backpressure in the exhaust system. The backpressure diaphragm opens and closes a small bleed hole in the main EGR vacuum circuit or diaphragm chamber. Opening the bleed hole reduces vacuum to the main diaphragm and prevents the valve from opening fully. Closing the bleed hole allows full vacuum to reach the main diaphragm so the valve can open wide and allow maximum EGR flow. With positive backpressure EGR valves, any increase in exhaust backpressure causes the EGR valve to open. This reduces backpressure somewhat, allowing the backpressure diaphragm to bleed off some control vacuum. The EGR valve begins to close and exhaust pressure rises again. The EGR valve oscillates open and closed with changing exhaust pressure to maintain a sort of balanced flow.

Negative backpressure EGR valves (1973 & up). The negative backpressure type of EGR valve reacts in the same way, except that it reacts to negative or decreasing pressure changes in the exhaust system to regulate EGR action. A drop in backpressure occurs when there is less load on the engine. This causes the backpressure diaphragm to open a bleed hole and reduce EGR flow. It's the same principle as with the positive type except that the control function occurs when backpressure goes down instead of up.


http://www.aa1car.com/library/egr.htm